This invention relates to an electrophotographic photoconductor for use in an image forming machine such as a laser beam printer, a facsimile, a digital copying apparatus. The present invention is also directed to an image forming apparatus, to an image forming method and to a process cartridge using the electrophotographic photoconductor.
Conventionally, many organic electrophotographic photoconductors using an organic conductive material have been developed and mounted in a large number of copying machines and printers. With rapid digitization of electrophotography in recent years, a demand for an electrophotographic photoconductor having characteristics corresponding to digitization is increasing.
In recent digital copying machines and printers, a reverse developing system is dominating. In a reverse development system, the charges on parts corresponding to black parts (colored parts) of a draft on the photoconductor are erased by exposure to light and a toner image is formed on the light-exposed parts, not on unexposed parts. When an electrophotographic photoconductor is used in a reverse development system, toner adheres locally in non-image parts and causes image defects such as black spots and surface stain. This phenomenon is caused by local neutralization of the charges on the photoconductor surface due to charge infection from a conductive support or a lower layer.
To prevent black spots and surface stain which take place at the time of reverse development, it is proposed to provide an undercoat layer for preventing charge injection from the conductive support or a lower layer between the support and a photoconductive layer (comprising a charge generating layer and a charge transporting layer). Such an undercoat layer needs to cause no adverse effects on the properties of the photoconductor even in repetitive use. However, with an undercoat layer made of a single resin material, it is difficult to realize this property. Also, in order to prevent charge injection from the conductive support or a lower layer, the thicker the undercoat layer, the better. However, it is very difficult to form a thick undercoat layer with a single resin material. Thus, a method in which conductive particles are dispersed in the resin for the undercoat layer is proposed.
In the case of photoconductors for use in laser printers or the like in which an image is written with a coherent light such as a laser beam, it is proposed to disperse a white filler having a high reflective index in the resin for the undercoat layer to prevent moire.
Also, as a method for preventing black spots and surface stain which take place at the time of reverse development, it is proposed to increase the thickness of the photoconductive layer to decrease the electric field applied to the photoconductor and not to allow charge injection from the conductive support or a lower layer.
In conventional reverse development systems, a corona charging system is employed. However, repetition of electrophotographic process using a corona charging system increases ozone and impairs the safety in use. Thus, in recent years, contact charging systems are used. A contact charging system generates much less ozone than a corona discharging system and thus causes no problem of environmental safety. However, a contact charging system has a peculiar problem of discharge breakdown caused by directly applying a high voltage to a photoconductor. In reverse development, discharge breakdown causes large black spots. Also, when the photoconductor is mounted in an image forming apparatus with a reverse development system, the absolute value of the potential of light-exposed parts increases, resulting in a decrease in image density.
To prevent discharge breakdown, it is necessary to increase the thickness of the undercoat layer to hide the defects on the conductive support surface such as flaws and unevenness. It is also effective to increase the thickness of the photoconductive layer to decrease the electric field applied to the electrophotographic photoconductor. However, such an increase of the thickness causes non-uniformity in image density of solid or half tone images.